selftests/bpf: Add bpf_percpu_obj_{new,drop}() macro in bpf_experimental.h

The new macro bpf_percpu_obj_{new/drop}() is very similar to bpf_obj_{new,drop}()
as they both take a type as the argument.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230827152805.1999417-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>